Method and system for machine vision-based feature detection and mark verification in a workpiece or wafer marking system

ABSTRACT

A system and method for inspecting machine readable marks on one side of a wafer without requiring transmission of radiant energy from another side of the wafer and through the wafer. The wafer has articles which may include die, chip scale packages, circuit patterns and the like. The marking occurs in a wafer marking system and within a designated region relative to an article position. The articles have a pattern on a first side. The method includes the steps of imaging a first side of the wafer, imaging a second side of the wafer, establishing correspondence between a portion of first side image and a portion of a second side image, and superimposing image data from the first and second sides to determine at least the position of a mark relative to an article.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional application Ser.No. 60/381,602, filed May 17, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to laser marking of workpieces, includingsemiconductor substrates, wafers, packages and the like. The inventionis particularly adapted for, but not limited to, marking machinereadable codes on a second side of semiconductor wafers which have ahigh density circuit patterns on a first side, for instance chip scalepackages having a high density of interconnects which could be damagedby a marking beam, or where space for codes is limited.

2. Background Art

The following representative patent references relate to various aspectsof laser marking of wafers and electronic assemblies, illumination, andinspection/reading marks: U.S. Pat. Nos. 4,522,656; 4,945,204;5,329,090; 6,309,943; 6,262,388; 5,929,997; 5,690,846; 5,894,530;5,737,122; and Japanese Patent Abstract 11135390.

The following representative references provide general information onvarious laser marking methods and system configurations and components:“Galvanometric and Resonant Low Inertia Scanners”, Montagu, in LaserBeam Scanning, Marcel-Dekker, 1985, pp. 214–216; “Marking Applicationsnow Encompass Many Materials”, Hayes, in Laser Focus World, February1997, pp. 153–160; “Commercial Fiber Lasers Take on Industrial Markets”,Laser Focus World, May 1997, pp. 143–150. Patent Publications: WO96/16767, WO 98/53949, U.S. Pat. Nos. 5,965,042; 5,942,137; 5,932,119;5,719,372; 5,635,976; 5,600,478; 5,521,628; 5,357,077; 4,985,780;4,945,204; 4,922,077; 4,758,848; 4,734,558; 4,856,053; 4,323,755;4,220,842; 4,156,124.

Published Patent Applications WO0154854, publication date 2 Aug. 2001,entitled “Laser Scanning Method and System for Marking Articles such asPrinted Circuit Boards, Integrated Circuits, and the Like” andWO0161275, published on 23 Aug. 2001, entitled “Method and System forAutomatically Generating Reference Height Data for use in aThree-Dimensional Inspection System” are both assigned to the assigneeof the present invention. Both applications are hereby incorporated byreference in their entirety.

U.S. Pat. No. 6,309,943 relates to identifying and determining aposition of a scribe grid on a front-side surface of a wafer with acamera. Based on this information, a laser is fired to form an alignmentmark on the back-side surface of the wafer.

U.S. Pat. No. 6,496,270, assigned to the assignee of the presentinvention, describes a method and system for automatically generatingreference height data for use in a 3D inspection system wherein localreference areas on an object are initially determined and then theheight of these local reference areas are determined to generate thereference height data.

The WH-4100 Laser Marking System is a commercially available backsidelaser marking system produced by the assignee of the present invention.A fine alignment vision subsystem corrects rotational or offset errors(X, Y, Angle) which are introduced when a wafer is placed in the markingstation. A manual “teach tool” allows the user to train the system torecognize three non-collinear points on the wafer that is to be used forthe correction. An iterative trial and error process with variousadjustments, and manual evaluation of the results is required with thesystem. The information is then used to determine mark locations on thebottom side of the wafer.

U.S. Pat. Nos. 5,894,530 and 5,929,997 relate to viewing systems usedfor inspection and/or alignment operations in microelectronics. In the'530 patent, optical elements are selectively positioned such thatimages of indicia fields disposed on either side of a substrate can beviewed (at the same magnification) whenever the substrate is in a givenorientation, or such that images of indicia fields disposed on bothsides of the substrate may be viewed at the same magnification,simultaneously.

With increasing density and complexity of circuitry on semiconductordevices, (e.g., exceeding ten-thousand die on a 300 mm wafer), multipledesigns and layout of circuitry, there is an on-going need to provideadvanced interactive tools for improving throughput and yield.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved method andsystem for machine vision-based feature detection and mark verificationin a workpiece or wafer marking system.

In carrying out the above object and other objects of the presentinvention, a precision laser based method of marking a semiconductorwafer having articles which may include die, chip scale packages,circuit patterns and the like. The marking occurs in a wafer markingsystem and within a designated region relative to an article position.The method includes determining at least one location from whichreference data is to be obtained using (a) information from which alocation of an article is defined, and (b) a vision model of at least aportion of at least one article. Reference data is obtained to locate afeature on a first side of the wafer using at least one signal from afirst sensor. The method further includes positioning a marking fieldrelative to the wafer so as to position a laser beam at a markinglocation on a second side of the wafer. The positioning is based on thefeature location. A predetermined pattern is marked on the second sideof the wafer using a laser marking output beam. The step of determiningincludes: measuring at least one feature in an image obtained from afirst wafer portion; relating the measured feature to a wafer map; andstoring the data for use when marking wafers substantially identical tothe first wafer. The steps of measuring, relating, and storing areperformed automatically.

The step of measuring may include measuring the average pitch of aplurality of articles and relating the average pitch to a wafer map.

The articles may comprise a row-column pattern of die, and the step ofdetermining may further include: locating a pair of orthogonal edges ofthe row-column pattern; forming bounding boxes from the edges and;defining a die pattern coordinate system from the bounding boxes.

The relative positioning of the wafer may be carried out in a primarycoordinate system substantially aligned with the movement of at leastone positioner. The method may further include transforming coordinatesto relate the primary coordinate system with the die pattern coordinatesystem.

The step of determining may still further include: obtaining acoordinate using a wafer map to provide the information from which alocation of the article is defined; and imaging at least a portion of anarticle on a first wafer to generate the vision model.

Still further in carrying out the above object and other objects of thepresent invention, a method is provided for inspecting machine readablemarks on one side of a wafer without requiring transmission of radiantenergy from another side of the wafer and through the wafer. The waferhas articles which may include die, chip scale packages, circuitpatterns and the like. The marking occurs in a wafer marking system andwithin a designated region relative to an article position. The articleshave a pattern on a first side. The method includes imaging a first sideof the wafer, imaging a second side of the wafer, establishingcorrespondence between a portion of first side image and a portion of asecond side image, and superimposing image data from the first andsecond sides to determine at least the position of a mark relative to anarticle.

The system may further include substantially matching images obtainedfrom the first and second sides so that the superimposed image portionscorrespond. The step of substantially matching may be carried out usinga calibration target and a matching algorithm.

The method may further include providing an input using the userinterface so as to cause a region of interest to be defined within atleast a portion of an image of an article.

The region of interest may be operator adjustable.

The superimposed data may be used to determine the position of a markrelative to the article.

The method may further include providing an inspection station having awafer positioning subsystem separated from a positioning subsystem usedfor marking.

Yet still further in carrying out the above object and other objects ofthe present invention, a precision laser based system of markingsemiconductor wafers, the wafer having articles which may include die,chip scale packages, circuit patterns and the like. The marking occursin a wafer marking system and within a designated region relative to anarticle position. The system includes means for determining at least onelocation from which reference data is to be obtained using (a)information from which a location of an article is defined and (b) avision model of at least a portion of at least one article. The systemfurther includes means for obtaining reference data to locate a featureon a first side of a wafer using at least one signal from a firstsensor. The system further includes means for positioning a markingfield relative to the wafer so as to position a laser beam at a markinglocation on a second side of the wafer. The positioning is based on thefeature location. The system still further includes means for marking apredetermined pattern on the second side of the wafer using a lasermarking output beam. The means for determining measures at least onefeature in an image obtained from a first wafer portion, relates themeasured feature to a wafer map, and stores the data for use whenmarking wafers substantially identical to the first wafer. Themeasuring, relating, and storing are performed automatically by themeans for determining.

Still further in carrying out the above object and other objects of thepresent invention, a system is provided for inspecting machine readablemarks on one side of a wafer without requiring transmission of radiantenergy from another side of the wafer and through the wafer. The waferhas articles which may include die, chip scale packages, circuitpatterns and the like. The marking occurs in a wafer marking system andwithin a designated region relative to an article position. The articleshave a pattern on a first side. The system includes means for imagingthe first side of the wafer to obtain an image, means for imaging themark on the second side of the wafer to obtain an image, means forestablishing correspondence between a portion of a first side image anda portion of a second side image, and means for superimposing image datafrom the first and second sides to determine at least the position of amark relative to an article.

At least one of the means for imaging may include a zoom lens.

The means for establishing correspondence may include a calibrationtarget and an algorithm.

Yet still further in carrying out the above object and other objects ofthe present invention, a laser based system is provided for lasermarking of substrates such as semiconductor wafers or similar substrateswith a laser marking beam. The substrates have a repetitive pattern ofarticles arranged in rows and columns. Each of the articles have afeature detectable with an imaging subsystem. The system has a lasermarking head, the imaging subsystem for imaging and measurement, amotion subsystem having a stage for positioning at least the substraterelative to the imaging subsystem, and a user interface connected atleast to the imaging subsystem and motion subsystem. Laser marks are tobe placed at predetermined locations relative to the articles. A methodof laser marking with beam position control using predetermined patternfeatures is further provided. The method includes providing, through theuser interface, an input so as to cause a portion of the pattern to beidentified for automatic feature detection and measurement with amachine vision algorithm. The method further includes positioning afirst substrate relative to the imaging subsystem automatically totraverse the pattern along at least one of a row or column of thepattern so as to acquire image data at a first set of feature locations.A dimension is measured using at least one detectable feature of aplurality of articles, the algorithm, and the image data. Dimensionaldata is stored based on the measurement. At least three featurelocations of a second set of feature locations are determined relativeto the pattern using the dimensional data. The feature locations of thesecond set suitably define a relationship between a pattern coordinatesystem and a stage coordinate system. The first substrate is removed anda second substrate is positioned to be marked relative to the imagingsubsystem. At least three corresponding feature locations of the secondset of feature locations are located in image data obtained from thecorresponding pattern on the second substrate. Coordinates of thepattern on the first substrate are related to the corresponding patternon the second substrate. The substrate is positioned relative to themarking beam based on at least the three feature locations of the secondset to mark the substrate.

A first estimate of the dimension may be obtained by semiautomaticrelative positioning of the substrate and the imaging subsystem over asubstantially small area of the pattern, and further includesidentifying a feature in a displayed image, and communicating the imagelocation of the feature using the user interface.

The substrate may be a semiconductor wafer, the articles are die, and afeature is a corner of the die.

The dimensional measurement may be the average die pitch measured over asubstantial number of die along at least one of a row and column.

The average die pitch may be related with a wafer map.

The pattern coordinate system may have an origin defined relative to aboundary of the pattern.

The step of determining may include searching for pattern locations, andsearching may be carried out by controlling the stage based on patternsystem coordinates.

The step of providing may further include generating a vision modelusing an image of a portion of the pattern.

The above object and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a view of the first side of semiconductor waferhaving articles, and a field of view covering several articles; lasermarking of each article is to occur in a corresponding field on thebackside of the wafer;

FIG. 1B shows an article of FIG. 1A in an expanded view;

FIG. 1C is a broken away expanded view of four articles within the fieldshown in FIG. 1A;

FIG. 1D illustrates exemplary two examples of circuitry which may bepresent on various articles, for instance a ball grid array and circuittrace patterns;

FIGS. 2A–2B shows several components of a marking system of the presentinvention with FIG. 2A showing the workpiece and exemplary optical andmechanical components, and FIG. 2B depicting a system controller;

FIG. 2C illustrates, by way of example (not to scale), ray diagramsassociated with non-telecentric alignment and marking systems,particularly as applied to backside wafer marking based on topsidefeatures;

FIGS. 3A–3C are a number of views wherein FIG. 3A shows a view of thesecond (bottom) side of the wafer with a marking field, corresponding tothe field of view of FIG. 1A, containing the articles of FIG. 1C; FIG.3B is an illustration, in a broken away view, of marks formed within adesignated region on the second side; and FIG. 3C shows an expanded viewof a marked article;

FIG. 4 shows an example of a galvanometer beam positioning system, whichmay be used in an embodiment of the invention for backside marking;

FIG. 5A is a schematic diagram showing certain subsystems of a lasermarking system for semiconductor wafers for use in a production system;

FIG. 5B is a schematic illustrating exemplary time efficient sequencingof operations for a wafer marking process;

FIGS. 6A–6B show two alternative beam positioners, which may be usedalone or in combination for laser marking;

FIGS. 7A–7D illustrates top, end, side, and perspective views,respectively, of a workpiece positioning mechanism for use in anembodiment of the present invention;

FIGS. 8A–8D are top, end, side, and perspective views, respectively,showing the use of two positioners of FIG. 7 for supporting andpositioning a rectangular workpiece (up to and including 2 degrees offreedom);

FIGS. 9A–9C are top, side, and perspective views, respectively, showingthe use of three positioners for supporting and positioning a roundworkpiece, for instance a 300 mm wafer (up to and including 3 degrees offreedom);

FIG. 10A is a schematic representation of an exemplary laser and opticalsystem for general wafer marking (e.g., topside marker shown);

FIG. 10B illustrates schematically degradation in mark quality (e.g.:due to cracking) with increasing laser penetration depth when comparedto a mark produced using a method and system of the present invention;

FIGS. 11A–11D relate to two and three-dimensional calibration of theworkpiece processing system of FIGS. 2A and 2B with various calibrationtargets;

FIGS. 11E–11J further illustrate various calibration targetconfigurations for calibrating various subsystems within a laser markingsystem;

FIGS. 12A–12C illustrate several features that may be located within afield of view on a first side of a wafer, the feature locations beingused to determine a position of a marking beam on the opposite side, forexample;

FIG. 12D illustrates coordinate systems and exemplary circuit featuresused for relating coordinates of a wafer to be marked with a storedrepresentation of the wafer;

FIGS. 13A–13C illustrate the design of a telecentric lens for use in aprecision wafer marking system with a deviation less than about 1 spotdiameter over (1) an 80 mm wide field, and (2) a depth rangecorresponding to nominal wafer sag and warpage specifications;

FIG. 14 illustrates schematically features of a laser mark on asemiconductor wafer;

FIG. 15 schematically illustrates a wafer positioning system wherein thewafer is initially loaded in a horizontal position, and moved to avertical position for alignment, marking, and inspection operations;

FIG. 16 shows a wafer holder capable of supporting wafers in horizontal,vertical, and upside down configurations; and

FIGS. 17A–17C show a calibration target and representative superimposedimage obtained with separate imaging systems so as to allow for markinspection and position verification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Overview

Several components of a system 100 for laser marking and inspection ofwafers, for instance 300 mm wafers, is schematically illustrated in FIG.5A. A robot 101 transfers a wafer from a FOUP (Front Opening UnifiedPortal) delivery device to a pre-aligner 102 which is used to find thenotch or flat of the wafer so as to orient the wafer for furtherprocessing. Reader 103 may be used to extract certain coded informationwhich in turn may be used in subsequent processing steps. A precisionstage 104 is used, and a fine alignment procedure included to correctthe residual error of the pre-aligner (e.g., X, Y, rotation). The waferis marked. All marks, or a designated subset, are then inspected. In thearrangement of FIG. 5A the inspection system is used with a separateinspection stage 105.

A marking sequence, following opening of a FOUP, includes:

-   1. Robot moves the wafer to the pre-aligner and establishes a    notch-die positional relation.-   2. The wafer ID is read by an OCR reader.-   3. Mark information is obtained from a network.-   4. The robot moves the pre-aligned wafer to a precision X-Y stage.-   5. Fine X-Y-Theta alignment of the wafer to a least correct residual    pre-aligner errors.-   6. The wafer is marked using a “mark-index field-mark -index field”    repeating sequence.-   7. The wafer is inspected.-   8. The wafer is returned to the FOUP.

FIG. 5B illustrates an exemplary sequence of operations for timeefficient wafer processing in a system. Various processing steps mayoccur in parallel. For example, a second wafer may be transferred forpre-alignment while fine alignment is occurring on a first wafer.

An exemplary 300 mm wafer may have several thousand articles (e.g.: chipscale packages, integrated circuits). The density of the circuitry oneach article can lead to difficulty in placing machine readable marks,such as 1-dimensional or 2-dimensional codes, in restricted areas. Forinstance, the die size on a 300 mm wafer may vary from about 25 mm to0.5 mm or smaller, with dense, complex circuit patterns. Further, damageto circuitry which might be caused by a high energy marking beam is tobe avoided. WO0154854, assigned to the assignee of the present inventionand hereby incorporated by reference in its entirety, discloses a methodof high resolution marking of electronic devices. Laser markregistration is obtained from circuit features measured with a sensor,and in one embodiment the sensor is located disjoint from a markinghead. Examples are included in '854 for marking of PCB multi-ups andpackages such as chip scale packages and die in a tray. Sections of the'854 disclosure, including: page 4, lines 9–16, page 6, lines 1–5 and22–29, Page 8, lines 10–17, page 9, line 15—page 10, line 30, page 11,lines 14–20 and the sections in the detailed description entitled “scanhead”, “marking operation”, and “registration” and the associateddrawings of the sections are related to the present disclosure andprovide additional support for various aspects of precision markingmethods and systems disclosed herein.

Referring to FIG. 1A, one embodiment of the present invention provides aprecision laser based method of marking a semiconductor wafer 3, and themethod may be adapted to marking of packages, substrates or similarworkpieces. The wafer 3 may have articles 2 (one shown in an expandedview in FIG. 1B) which may include die, chip scale packages, circuitpatterns and the like. The articles may be substantially identical, butsuch a restriction is not necessary. In a typical semiconductormanufacturing operation, subsequent to marking, the articles will be theseparated by precisely cutting (“dicing”) the wafer. Further informationmay be found in U.S. Pat. No. 6,309,943 wherein alignment marks 35 (seeFIGS. 3A–3C) placed on the back of a wafer are used to define a path forprecision cutting. Referring to FIG. 3, the marks 36 on an article areto be formed within a designated region 30 relative to an articleposition. In this example the backside 33 of the wafer 3 is marked neara corner of the article. Circuits 34 correspond to a backside view ofcircuits 4.

In a preferred embodiment for marking large (e.g.: 300 mm) wafers acalibration process will be used to relate an alignment vision systemcoordinate (e.g., a “first side” position, for instance at the sensorcenter position, and at best focus) and beam positioning sub-systemcoordinate (e.g.: laser beam waist position at the center of a markingfield). Preferably, the calibration will provide three-dimensionalcorrection. The increasing demand for precision placement of marks inlocalized areas over a large field lead to increasing beam positioningaccuracy and decreasing spot size requirements for obtaining finer linewidths or character sizes. Over a large workpiece “sag” and warpage maybe significant relative to the depth of focus, which introducesconflicting design parameters. Preferably, a laser spot size can beadjusted during system operation while maintaining spot placementaccuracy. With reference to the arrangement of FIG. 2A, one embodimentincludes calibrating a first sensor sub-system 14 (e.g., a “alignmentvision system”) and a beam positioner sub-system 19 (e.g., “markinghead”). The calibration is used to relate a first side position and amarking beam position, the sub-systems each having a field of view whichis a portion of a workpiece 11 to be marked. The workpiece may be asemiconductor wafer 3. Further details regarding various steps of acalibration process providing both 2-dimensional and 3-dimensionalcapability are provided in SECTION 1 which follows entitled “2D/3DCalibration.” Further details on various calibration procedures forworkpiece processing can be found in exemplary references (1) U.S. Pat.No. 5,400,132 entitled, “Rectification of a Laser Pointing Device,” (2)U.S. Pat. No. 4,918,284 entitled “Calibrating Laser Trimming Apparatus,”and (3) WO0064622, “Laser Calibration Apparatus and Method.”

FIG. 2C illustrates, by way of example, the multiplication of beamposition error with depth in a non-telecentric system when marking awarped wafer 143 on the backside using frontside data (though not sorestricted). The wafer has a thickness 146, which is typically at leasta few hundred microns. Topside alignment camera 142 is shown, for thepurpose of illustration, to be aligned with marker head 147 alongoptical centerline 149. Planes 148,144 represent reference planescorresponding to working distances from the marker and camerarespectively. In absence of depth variations, these planes intersectcamera viewing rays and marking beams at wafer surface positions.Reference data along ray 140 is obtained from a reflection at the wafersurface at the point of intersection of the wafer. The data will be,without correction, represented as a coordinate corresponding to theintersection with plane 144, which is to be related to a markingcoordinate. A lateral position error 1400 results. Assume for thepurpose of illustration a mark is to be placed on the back of the waferat a position corresponding to reference data taken along ray 140 at thewafer intersection. A marking beam, without correction, will be directedto a point in the plane 148 corresponding to the reference data (andposition error). However, this may result in a mark outside of adesignated region, as shown by the direction of central ray of markingbeam 141 at the actual intersection point with the wafer.

The three-dimensional calibration process of SECTION 1 of the Appendix,with suitable height measurements of the wafer, may be used to determinea correction to be applied to the beam positioner.

In a preferred telecentric system, the error is reduced to about 1 spotor finer with a lens (see SECTION 5 which follows entitled “PrecisionTelecentric Lens”) of low to moderate cost. Preferably, the telecentricdesign compensates for the worst case wafer warpage and additionalsystem “stackup” errors. With the preferred arrangement a field sizesupporting relatively high marking speeds is maintained. In thetelecentric case the calibration process may be streamlined, butmultiple calibration files used to at least control and maintain thelaser spot size over the working volume are preferred. This provides forconsistent marks and for mark contrast control.

Three-dimensional tolerances are to be considered for the alignment andmarking sub-systems in view of the workpiece variations relative to thedepth of focus of the optical systems. Increasing the alignment systemmagnification to improve feature location accuracy decreases the depthof focus. Various focusing methods are useful to position the entiresub-system 14 and/or lens system 15 (shown as a telecentric lens but notso restricted) relative to the workpiece along the Z-axis. For example,the Z-axis position corresponding to the maximum edge contrast at a dielocation is a possible measure. A measurement of the maximum intensityof a “point” or small target (one the order of a pixel) may provide moresensitivity to depth changes.

Wafer “sag” is somewhat predictable from a specification of waferthickness. Predictions based on models (fixed edge and simple support)with wafer thickness ranges of about 300 μm to 775 μm indicated about 60μm of deviation for the latter case. For thinner wafers the deviationincreases, and the overall deviations may be further increased bywarpage and other stackups. Surface deviations may be estimated and usedfor certain correction. A telecentric system, for instance as describedin SECTION 5, is predicted to yield less than 1 μm of spot placementerror over a 4″ marking field. Various sub-systems, including the scanhead, alignment vision system, and perhaps inspection system may includeat least an option for height sensing. Similarly, a separate sub-systemcould be added specifically for height measurements at a plurality oflocations on the wafer surface. Preferably, any degradation in the cycletime of the machine will be negligible.

In one arrangement the alignment vision system 14 will be relativelypositioned at sample points which may include but are not limited to theregions used for feature detection. As mentioned earlier, the focussensing may be achieved by sampling the image contrast at locationsalong the z-axis using the alignment vision system. The z-axis locationsare recorded. Alternatively, a triangulation or focus sensor, which maybe a commercially available module, may be used for measuring surfacepoints which are used with the alignment and calibration algorithms (andthe known wafer thickness) to map the surface. Similarly, a directmeasurement of the second side may be obtained with a sensor includedwith the vision inspection module 20. In an alternative arrangement a“full field” system, for instance a commercially available Moire Camera,may be used. In any case, the data will preferably be used to positionthe marking beam waist at the surface. In accordance with the preferredcalibration method of SECTION 1, the desired spot size will bemaintained at the marking locations. In one arrangement the marking beamwaist may be positioned in discrete steps, for instance at 9 locationswithin an 80 mm field for center-edge compensation. Non-contact opticalsensing is preferred, but capacitance or touch probes may be acceptable.

If the deviations correspond to a simple second order curve and aresymmetric, then the wafer surface may be sampled along a diagonaldirection using at least three locations (edge region, center, edgeregion). If warpage is represented with a higher order curve (e.g.:“potato chip”) additional data will be acquired, for instance at leastnine locations. If the data is acquired with the first side alignmentsystem, the second side location may be approximated using the thicknessof the wafer, which may be measured or specified by the operator.

Similarly, both for calibration and marking, a marking beam focusfunction may be sampled at a number of locations in the marking field(at reduced power). The system may include a detection system suitablefor measuring “featureless” surfaces, for example a bicell or quad-cellarrangement. Alternatively, a projected grid may be used similar to theoptions provided in commercially available Metrology equipmentmanufactured by Optical Gaging Products (Rochester, N.Y.). The focusingtool will preferably be used for both alignment and system setupoperations in addition to measuring-the working distance during wafermarking.

In the system of FIGS. 2A and 2B, both the beam positioning subsystemand the alignment system preferably include telecentric optical systems351 and 15, respectively, which reduce or eliminate variation in theposition of an angular scanned marking beam position with depth. SECTION5 shows a telecentric lens system which provides spot placement accuracybetter than one spot diameter over a field size of about 80 mm, and overa depth range corresponding to worst case expected sag/warpage. The 80mm field allows for significantly higher marking speeds compared tosmaller non-telecentric fields. Furthermore, the 30 μm spot size isfiner than most wafer mark systems, a desirable feature for controllingmark contrast and resolution.

However, other alternatives may be used with appropriate compensationfor positioning with depth. For instance, in one embodiment atelecentric lens 15 may be used, but an arrangement similar to 47 ofFIG. 6 may be used for marking (as discussed below).

With reference to FIG. 2A, the preferred alignment sub-system will havea high resolution camera 13, for example a 1280×1000 CCD imaging arraywith image processing hardware and software for extracting andprocessing smaller regions using a “software zoom” feature.Alternatively, a calibrated “zoom” optical system may be used.Illumination system 21 may include special illumination design, forinstance a combination of dark and bright field illuminators, to enhancethe contrast of features used for alignment. In one embodiment an LEDarray provides low angle illumination, with a manually adjustable angle.In the configuration using the high resolution camera the exposure isfixed which simplifies the design, eliminating the dependence of theimage “brightness” with magnification.

In one embodiment, the marking sub-system 19 includes the system shownin FIG. 4 with X-Y galvanometers providing deflection system 40, 41, 42,43 and possibly a beam expander assembly 49. FIG. 6, incorporated fromthe earlier cited reference to Montagu, pp. 227–228 shows alternativepre-objective 46 (e.g.: telecentric) and a post-objective 47 scanningarrangements, the latter incorporating an additional dynamic focustranslator 48. In a preferred telecentric system of the presentinvention components may be included for dynamic focus 48 and/or spotsize adjustment with a computer controlled version of expander 49 ofFIG. 4.

The fine alignment system provides correction for residual X-Y-angleerrors associated with the transfer and pre-aligner. In one embodimentwherein only small variations occur or are specified, the alignmentsystem may correct X, Y, and theta (e.g.: angle) variations withmeasurements taken at three locations (e.g. fiducials). However, withemerging tight tolerances an increasing density of circuit/wafer,increased accuracy is preferred. The fine alignment system of 14provides added capability of recognizing and/or measuring featuresassociated with an article 2 of the wafer (e.g.: machine vision/patternrecognition capabilities). A feature location will be determined. Analgorithm is used to obtain reference data and to locate a featureassociated with at least one article 2 on a first side of the workpiece3 using at least one signal from the first sensor 13. For example,article 2 of FIG. 1A, shown in an expanded view of FIG. 1B, may have acircuit pattern with detectable conductor traces 7 or pads 5 which maybe replicated 4 in at least a portion of the wafer (but not necessarilyover the entire wafer). In a preferred system, a pattern recognitionalgorithm will, based on “training” on a reference wafer, for instance,automatically learn at least a portion of the workpiece structure anddetermine the relative location of the pads, traces, or similarfeatures. For instance, the rectangular outline of a die (article) 6 orcorner locations may be used as one feature to locate the die edgeand/or estimate the center. The location may be related to a location ofat least one other die in 4 located within the marking field 1 of FIG.3A, or possibly outside the field if tolerances permit. For example, aminimum of 3 non-collinear locations are determined over the workpieceand used to calculate an offset and rotation correction for the entireworkpiece. Another pattern may be defined by the location of an array ofsolder balls or pads 8 as an alternative/equivalent. Yet another patternmay include sections of internal circuitry of the article having evengreater density than illustrated in FIGS. 1A–1D. The algorithm mayinclude matching features of the workpiece using a machine visionsub-system, for instance a grey scale or binary correlation algorithm.Various “modules” and algorithms for pattern recognition and matchingare commercially available (e.g.: Cognex Inc.) which may be adapted foruse with the present invention. The workpiece may have identical andrepetitive patterns.

In a preferred arrangement the matching is automatically performed overall the articles, and without human intervention. It should be notedthat many combinations of patterns may be present on a wafer withspecial marking requirements (e.g.: “binning”) and the preferredalgorithm will have substantial flexibility. The “training” may furtherinclude a semi-automatic, operator guided teaching phase so as toefficiently program the machine for recognition and matching of complexpatterns.

In WO 0161275, incorporated by reference and assigned to the assignee ofthe present invention, various detection and recognition algorithms aredisclosed for automatic learning of circuit features using grey-scaleand/or height information, and subsequent use of the stored informationfor inspection. For instance, the following sections of the '275disclosure: page 7, lines 4–26; page 8, lines 1–5 and lines 17–25; page9, lines 5–10; page 10, lines 24–25; page 11, lines 1–18; page 15, lines29–30; page 16, lines 1–10; page 17, lines 19–28 and the associateddrawings teach the application of various pattern recognition andlearning algorithms. Further details of various steps for detection andmatching features for obtaining reference locations for precisionmarking of wafers and similar articles are disclosed in SECTION 2 whichfollows entitled “Feature Detection and Fine Alignment.”

In one embodiment of a 300 mm marking system, an 80 mm marking field isused for high speed, and an alignment vision field of approximately 16mm is used to for feature detection. With a 1024×1024 array a 16 μmpixel size will be provided, which is somewhat finer than the spot sizeof the marking beam. For example, in an embodiment of a backside wafermarking system a spot size of less than 40 μm is preferred, with a mostpreferred range of about 25–35 μm. The marking field 1 dimension(depicted in FIG. 3 and corresponding to the region 4 of FIG. 1-A but onthe backside 33 of wafer 3) may be a relatively small fraction of theworkpiece 3 dimension (e.g.: a 300 mm maximum wafer size in a systemconfigured so as mark wafers of varying specified dimensions). Forexample, in one embodiment for marking 300 mm wafers nine or moremarking fields having dimensions in the range of about 75–100 mm areused to provide marking precision and high speed operation. In a casewhere a workpiece is severely warped, the marking field may be reducedby controlling the amplitude of the scan angle, based on surfacemeasurements or a specification. Precision marking includes relativelypositioning the beam positioner sub-system 19 (or a component of thesub-system) and the workpiece 11 so as to position a laser beam at amarking location 30 on a second side of the workpiece 33 as shown inFIG. 3, the positioning based on the feature location on the first side.The feature location may define the location of the article (e.g.: edgeor center) or otherwise be related to designated region(s) 30 formarking located on the second side. Various methods and sub-systems maybe used for the positioning as described in more detail below.

As shown in FIGS. 3A–3C, a predetermined code or other machine-readableindicia 36 is marked on the workpiece, typically with a scanned lasermarking output beam (vector or dot matrix, for instance) within thefield defined by 24 of FIG. 2A, preferably using telecentric lens 351. Amachine readable mark is formed in the designated region. Also, laserinduced damage to an article 2 is avoided by marking the second side

The steps of obtaining reference data, relatively positioning, andmarking are repeated so as to locate a feature associated with at leastone article on the first side, and to position a marking beam within allthe designated regions on the second side based on the featurelocation(s).

The beam positioning sub-system preferably includes a 2D galvanometerscanner 40, 41, 42, 43 as shown in FIG. 4 (but preferably adapted forirradiating the workpiece with a telecentric beam as shown in FIG. 2Aand approximately as in arrangement 46 in FIG. 6). Alternatively, thesub-system may include a translation stage or rotary stage with beamdelivery optics. The laser and optical system may be integral orremotely coupled, for instance with a fiber delivery system. The fieldof view of the beam positioner may range from a few laser spot diametersto a relatively wide angular field, but for precision marking inaccordance with the present invention the field will be a portion of thelargest workpiece to be marked in the system. For example, wafers of100, 200, and 300 mm may be marked and the marking field 1 dimension(e.g.: first side view in FIG. 1A, second side view in FIG. 3A) may beabout 100 mm. In certain cases a pattern may be marked on workpiece (saywith a lower laser power requirement) with parallel beams as illustratedin publication WO961676, and/or U.S. Pat. No. 5,521,628. Variouscombinations of serial and parallel operation may be used, for instancewith multiple marking heads as taught in U.S. Pat. No. 6,262,388. The2D/3D calibration process of the present invention may be adapted tothese marking configurations to maintain accuracy.

Relatively positioning may further include: (i) providing a beampositioner which may include a 2D galvanometer deflector; (ii) adjustinga mirror 42, 43 position (See FIG. 4) if the marking location is withinthe field; (iii) relatively translating the workpiece 11 and beampositioning sub-system 19 so as to position the location within themarking field 1 whenever the location is outside the marking field. Thefeatures related to article 2 (also depicted by the dashed lines of FIG.2A) are used as discussed above to determine a position of the markingbeam, and the position will preferably be a three dimensionalcoordinate. Further, the specified or measured thickness of the wafermay be a parameter used to determine the focal position of the beamrelative to a front side position.

In a preferred system for wafer marking at least one workpiecepositioner is used in addition to stage 18 (also depicted as 104 in FIG.5A) for fine positioning. The positioning sub-system is configured so asto support and position workpieces 11 of varying specified dimensions,while allowing radiation beams (marking beam(s) over field 24 andillumination/viewing beams in fine alignment camera field 25 from lightsource 21) shown in FIG. 2A to directly irradiate the first and secondsides of the workpiece. In one embodiment, a wafer chuck 17 (see SECTION3 which follows entitled “Workpiece Chuck/Positioner”) is provided witha Z-axis (direction 26) drive with an option of smaller wafer inserts tosupport the wafer or other workpiece. The system is preferably automatedwith an arrangement of end effector(s) transferring the workpiece to thechuck 17 which automatically clamps, grips, or otherwise supports (shownin a single schematic view in FIG. 2A) the workpiece. Surface damage andsignificant distortion are to be avoided.

In view of the aforementioned emerging three-dimensional variations andtolerance requirements, it is preferred that the marking beam focusposition shown as 422 in FIG. 4 (e.g.: beam waist) and attitude (roll,pitch relative to the focal plane) depicted by the arrow 22 (see FIG.2A) be adjustable. For example, variations in “sag” or warpage of thewafer in addition to stackup tolerances may be compensated by providinga total adjustment range of at least about + or −2 millimeters.Referring to FIG. 2A, the adjustment may include relative Z-axis (depth)positioning of the laser beam positioning sub-system 19 and workpiecealong a direction substantially perpendicular to the workpiece so thatthe beam waist of the laser substantially coincides with the workpiece.The adjustment may be dynamic and done for each wafer. The adjustmentmay include tilting 22 (pitch, roll) of the laser beam positioner and/orworkpiece to so that a focal plane of the laser beam is substantiallyparallel to a local planar region of workpiece (e.g.: over a markingfield). Alternatively, a planar region may correspond to a best fitplane over the workpiece. Some adjustments may be done with acombination of manual or semi-automatic positioning of the beampositioner, for instance during calibration or setup. Similarly, the endeffector(s) and the chuck 17 coupled to precision stage 18 may becontrolled by a program so position the workpiece 11 in angle (roll,pitch) and depth. SECTION 3 of the Appendix illustrates specific detailsof an embodiment for automatic precision positioning of a circular (forinstance a 300 mm wafer) or rectangular workpiece with actuators foradjustment of the height and preferably attitude. The arrangement isparticularly adapted for height adjustment. Various modifications, forinstance spherical or point contact at the support base 53 in FIGS. 7Cand 7D, will facilitate the fine angular positioning (roll, pitch) ofthe workpiece, for instance, tilting wafers having thickness of 300 μmor less.

In an alternative arrangement the wafer may be held in a verticalposition. For instance, a suitably modified and automated version of the“Wafer Edge Fixture” produced by Chapman instruments, and configured fora maximum wafer size 300 mm (Chapman Instruments, Rochester, N.Y., andreferenced to U.S. Pat. No. 5,986,753) may be used. Six degrees offreedom are included for profiling of wafers. Further description of thetilt stage, wafer chuck, X-Y-Z stage, and controller are found in thearticle “Wafer Edge Measurements—New Manual Fixture Provides MoreFeatures.”

In one embodiment for wafer marking a “split gantry” stage is analternative with automatic positioning of the horizontal mounted markinghead along one direction (e.g.: “X”, horizontal, into the page) andwafer positioning in at least a second direction (e.g.: “Y” vertical andalong the page, and “Z” along the optical axis, and preferably includingcapability for roll and pitch adjustment).

FIG. 15 illustrates a perspective view of yet another positioningarrangement with several components marking system also illustrated. Thewafer is translated in two dimensions (e.g.: translation in a planeperpendicular to the page of FIG. 2A). The wafer is oriented with an endeffector to notch 702 and loaded into holder 701. A hinge 703 is usedfor loading in the horizontal position followed by transferring to avertical position for marking with a beam incident through scan lens351. At least two axes of motion 704 and 705 are provided. Theconstruction allows for marking the backside and for fine alignmentusing camera 13 wherein the location of front side features are used toposition the marking beam.

FIG. 16 shows details of one arrangement for holding wafers at variousorientations. In this arrangement wedge 800 is engaged by a spring 801held open by vacuum so as to allow for mounting in a horizontal,vertical, or upside down orientation.

Various combinations of the motion (manual or automatic) of the (1)workpiece positioner 18 and (2) beam positioning sub-system (e.g.:“marking head”) 19 and/or (3) internal components of 19 (e.g.: a dynamicfocus sub-system 48 and/or beam expander 49 may be used and coordinatedwith controller 27. For instance, five axes of motion (e.g.: X, Y, Z andRoll, Pitch) may be implemented for precision positioning in a waferprocessing station 100. Further, coarse (possibly manual orsemi-automatic) positioning may be implemented in one or more axes, forinstance.

The selection of laser pulse characteristics can have a significanteffect on the speed, contrast, and overall quality of the marks. Forbackside marking of Silicon wafers a pulsewidth of about 15 ns,repetition rate of about 25 KHz, and output energy of about 0.23–0.25millijoules at a wavelength of 532 nm provided favorable results. Ashort cavity green Vanadate laser was used. Further, marking depthpenetration of about 3 μm–4.5 μm provided machine readable marks withoutinternal damage (e.g.: cracking) of the wafer. Marking speeds of about150 mm/sec were achieved, and it is expected that about 350 mm will beachievable with preferred laser parameters. The marking speed representsa relative improvement for marking in view of the large number ofarticles to be marked at high resolution. An exemplary range ofoperation includes pulse width of about 10–15 ns, repetition rate ofabout 15–30 KHz, with focused spot size of about 30–35 μm for marks onSilicon wafers. Another range may include a pulsewidth of up to about 50ns, and a minimum repetition rate of about 10 KHz. Micro-cracking isalso prevented by limiting penetration of the beam to a depth of lessthan about 10 μm. It is expected that a wavelength of 1.064 μm will besuitable for marking metal workpieces, with frequency doubled operationfor Silicon wafer marking. Further details on a preferred laser andassociated characteristics are disclosed in SECTION 4 which followsentitled “Laser Parameters and Mark Quality.”

Referring to FIG. 2A, a vision inspection system 20, viewing the secondside, will generally include an illuminator, camera or other imagingdevice, and inspection software. In a preferred system the inspectionfield is calibrated to the fine alignment vision field. For instance,the centerlines may be aligned 29 as shown in FIG. 2A, with a largeoverlap between the fields. This provides for overlaying the marks onthe die for mark manual or automatic visual verification. SECTION 6which follows and is entitled “Backside Mark Inspection With FrontsideDie Registration” describes details of an embodiment for inspectingmarked wafers. All the marks (100% inspection) may be inspected, or auser-specified subset. For example, a few locations on the wafer may bemarked and the results analyzed. If the results meet specifications allthe remaining designated regions of the wafer may be marked. The visionsystem may be mounted on a separate stage wherein a first wafer isinspected while a second is marked (See FIG. 5A). FIG. 2A illustrates analternative arrangement wherein a single stage 18 is used to positionthe workpiece for both inspection and marking.

The inspection system will preferably provide feedback regarding markquality as rapidly as possible to maximize yield. For instance, a wafermay have 30,000 chip scale packages as articles. A marking field mayhave at least a thousand die. A separate inspection system with“standard” lighting for viewing marks may be an advantage to establishcorrelation between various stages of the wafer and device assemblysteps wherein the marks may also be viewed. In an embodiment where theinspection system optical axis is separated the inspection may occur ina sequence where a first field is marked and then inspected. Theinspection of the first field will occur while a second adjacent fieldis being marked when a large number of articles are to be inspected.

In an embodiment using a pair of galvanometer mirrors, data representingat least a sample of die (or other article) over the field may beacquired with a “through the lens” vision system (e.g.: a second simplervision system for the case of wafer mark inspection). The dataprocessing operation may overlap with positioning (indexing) to anadjacent field. It should be noted that the coaxial vision system mightnot require a vision system with complete inspection capability. Forinstance, the intensity or radiation pattern of the reflected scannedbeam may be analyzed for early detection of gross mark defects or otherprocess problems. For instance, a single photodetector may be used toanalyze the reflected marking beam. Telecentric viewing (e.g.: receivedthrough lens 351) reduces variations with angle, which can provide forimproved classification of signals.

Some Further Discussion of Various Alternatives:

In a preferred embodiment the workpiece 11 is translated when indexingto marking fields. However, the relative motion of the workpiece 11 andbeam positioning sub-system 19 may include translation of at least aportion of the beam positioner (or a component). When marking wafers, asingle X-Y stage moving the wafer allows for positioning of thealignment system 14, marking lens 351, inspection system 20, andpossibly an optional mark verification reader. In an embodiment whereinthe wafer is translated, alignment and beam scanning may be simplified.In an embodiment where the positioning sub-system or portion of thesub-system is translated fiber beam delivery from a remote laser sourceto marking head 19 may be used to an advantage.

In one embodiment for wafer marking a Z-axis stage 28 may be used. Arange of at least + or −2 mm is preferred. The beam positioner 19 andlens 351 may move, but movement of the wafer is preferred. The Z motionmay be determined by the focus of the alignment camera system components13, 15. The sag and warp of the wafer is preferably compensated bymovement (translation, roll, pitch) of the wafer with the positioningsystem 18, 17 or by movement of the beam positioner 19 as describedabove.

A total Z range of travel of about 12 mm, implemented with one or moretranslators, may be used to allow a robotic end effector to load a waferwhile allowing for compensation of wafer sag by relative movement of thewafer and marking beam focus location.

A method for controlling contamination may be an advantage. For example,a tilted window, placed between lens 351 and the workpiece, with aslight amount of vibration may remove particles from the marking lens.Air pressure may be used to clean the lens during idle periods. A tiltedwindow will displace the beam and aberrations may be introduced. Certainerrors (e.g.: beam displacement) may be corrected during calibration.Alternatively, an “air knife” may be used to produce fast moving airacross the lens.

An exemplary exclusion zone of about 2–3 mm is typically used.

The wafer nest may have vacuum applied on the 2 mm exclusion zone. Thenest may be held with a kinematic mount.

The focal position of the alignment system lens 15 and camera 13 may beused for determining a Z-axis location and for fine positioning of thebeam. In one embodiment the wafer is translated. Alternatively, thecamera system may be focused and the position recorded. The position mythen be related to the beam positioner coordinates (e.g.: the lensposition) and the lens and positioner translated accordingly.

In one embodiment slight relative movement of the Z-axis position may beused to compensate for sag and warp. For instance, a change in thez-axis position may be effected at a plurality of marking locations overa 100 mm marking field. For instance, Z-translation may occur at ninelocations (e.g.: to compensate from center to edge).

The X-Y table may have a range of travel of about 12–18 inches, withlinear encoders for position feedback.

An inspection module may have optical resolution of about 4 microns.

A telecentric lens may be used with the fine alignment system.

The inspection module 20 may also be used for certain alignmentoperations (e.g. locating a fiducial on backside) and may be calibratedusing a transparent alignment target to establish correspondence withthe coordinate system of the fine alignment camera 13.

The recommended marking depth for optimum reading, while avoidingsubstrate damage, may be about 3.5 microns. The laser system may beconfigured for a maximum mark depth of about 10 microns.

Embodiments of the present invention may be used to mark wafers withprogrammable field sizes and number of fields (e.g.: 9–16 fields of viewon a wafer having a diameter in a range of 150–300 mm), focusing options(e.g.: 3 focus positions for wafers 775 microns thick with increasingdensity for thinner wafers), and various marking speeds (e.g.: 150–250mm/sec).

Various exemplary and non-limiting system parameters and associatedtolerances may include:

PARAMETER TOLERANCE Encoder Resolution .1 microns Z-stage Travel 10 mmZ-stage Perpendicularity .1 mRad Z-atage Accuracy +/−5 microns FineAlignment Repeatability 1–2 microns Spot Size ≦60 microns nominal, 25–40μm preferred Galvo (calibrated field) +/−30–50 micron accuracy MarkingLens Option (due to sag) telecentric, +/−3 micron, 300 micron waferthickness, 300 mm wafer Marking Lens Option flat field, +/−10 micron,775 micron wafer thickness, 300 mm wafer

Numerous alternatives may be used to practice the invention. Variationsof the positioner type, number of positioners, vision systems, focusinghardware, laser types including q-switched and fiber lasers, may beused. Furthermore, the choice of serial/parallel operation of multiplemarkers and inspectors for efficient production time management andyield improvement, including cluster tools and statistical processcontrol may be incorporated for use with a precision marking system ofthe invention. Further, it is contemplated that the pattern recognitionand marking techniques of present invention may be used alone or incombination with other production processes, for instance the “dicing”operation described in the aforementioned '943 patent.

SECTION 1—2D/3D Calibration

Various commercially available marking and workpiece processing systemscalibrate the laser marking field by marking a grid on test mirror andmeasuring the grid on a separate coordinate measuring or metrologymachine. It is an iterative process and very time consuming. Other lasersystems use the on-line through-scan-lens vision system to calibrate thelaser-marking field on the same side. Alternatively, a substrate ordisposable workpiece may be marked.

In accordance with the present invention, “two-dimensional calibration”utilizes an x-y stage, a pair of stages translating the workpiece and/ormarking head, or other arrangement which allows the on-line machinevision sub-system 14 of FIG. 11A to calibrate the laser marking field 24on the OPPOSITE side. The calibration is used to mark the second sidebased on vision data and features from the first side.

Calibration may be system dependent and manual, automatic, orsemi-automatic. By way of example, four steps for calibration are shownbelow to illustrate aspects of overall system calibration:

-   1. Calibrating camera pixels for each camera in system.-   2. Calibrating coordinates of a first camera to a second camera.-   3. Calibrating stage coordinates to camera coordinates-   4. Calibrating the scan head to the wafer nest.

FIG. 11E schematically illustrates a typical arrangement for respectivetop and bottom cameras 501 and 502. In at least one embodiment of thepresent invention each camera is calibrated separately to match thecamera pixels to actual “real world” coordinates. FIGS. 11F and 11Gschematically illustrate a “tool area” 505, which is relativelypositioned within camera 501,502 fields of view. Preferably, the camerasmay be mechanically positioned within the system so the fields of viewsubstantially overlap, but the fields may be separated. In one exemplaryarrangement the crosshairs 506 may be about 5 mm apart. The calibrationmay include measuring the coordinates of the crosshairs and estimating acenter position, scale factors, and rotation of a coordinate systemrelative to the tool. Preferably, at least the “pixel size” of thecamera will be measured. Alternative embodiments may include additionalcrosshairs of other suitable targets and calibration of sub-fieldswithin the camera field of view.

FIG. 11H illustrates a calibration step wherein the top and bottomcameras preferably view (simultaneously) target 511 as seen by a firstcamera and the same target depicted by dashed lines 510 as seen by asecond camera. The calibration target may be within the “tool area” asshown. A correction for offset, scale, and rotation is applied. In oneembodiment an additional crosshair may be used to specify the center ofthe object. This arrangement, with precision calibration, isparticularly useful for providing a display showing a mark on thebackside of a wafer relative to a die position as seen on the front sidefor the purpose of mark inspection (see SECTION 6).

Yet another calibration step may be applied to compensate for X-Y stagetolerances. FIG. 11I illustrates three crosshairs 520 used forcalibration wherein the entire nest is moved and camera coordinates arerelated to stage coordinates. As such, the tolerance stackup of thestage is compensated.

Yet another calibration may be applied to calibrate the scan lens 351 ofmarking head 19 in FIG. 11A to stage coordinates. FIG. 11J shows aconsumable part, for instance a black anodized disk 521 which may bemarked with five crosshairs, one shown as 522. Software is used toinspect the marked plate. The marking field may be a fraction of thedisk 521 size, and an X-Y stage provides for relative positioning of thedisk and marking beam.

These basic steps above may be sufficient and preferred in a systemwherein marking performance is substantially invariant with depth (e.g.:large depth of focus, relatively large laser spots, relatively smallwafers having exemplary thickness of about 775 microns and minimal sag).

In one embodiment the alignment vision subsystem 14 of FIG. 11A may becalibrated first with a previously marked wafer or alternatively with aprecision grid (e.g.: each preferably conforming to a calibrationstandard). For instance a 200 mm wafer or other maximum wafer size to bemarked with the system may be used. The wafer marks may include with agrid of fiducials similar to a crosshair 522 of FIG. 11J. In oneembodiment the wafer has a 77×77 array of crosshairs with 2.5 mm spacingwith a special pattern at the center of the grid. The camera focus ispreferably checked (e.g.: contrast measurement) over the grid andmechanical adjustments made to the nest. Alternatively, a positioner(e.g.: see FIGS. 9A–9C) may be adjusted in depth or attitude if used ina system. The marked calibration wafer is also used for a nextcalibration step wherein the X-Y stage 18 is calibrated. The initial X-Ystage calibration may take several hours to complete with calibrationover the range of travel, the calibration information being recorded byimaging a crosshair or other suitable target on the calibration wafer.The data is then evaluated. A third calibration step of the embodimentis a marking field calibration wherein a 200 mm wafer (or maximum sizewafer to be marked) is marked with a pattern similar that of FIG. 11J,or other pattern with suitable density. Preferably, the X-Y stage iscalibrated as above prior to calibration of the marker. The markpositions are then measured with using the fine alignment camera, orwith a separate vision subsystem. For example, the marks may be measuredwith a commercially available, “off-line” precision Metrology systemproduced by Optical Gaging Products (OGP), for instance a Voyagermeasuring machines. If marker field calibration is to be periodicallyrepeated as part operation of the marker in a production environment,the alignment vision system may be used. Preferably, the resolution andaccuracy of the alignment system will substantially exceed the minimummark spacing.

Compensation for workpiece sag and warpage may require maintaining thesame spot size with different working distances. Besides, there is anincreasing need to change laser beam spot sizes during operation to meetdifferent application parameters, such as line width, character size,mark contrast, hard-mark, soft-mark, throughput, etc. Three-dimensionalcalibration provides calibration at a plurality of marking positionsalong the Z-axis. As a result, the laser marking field capability isprovided for changing the laser beam working distance and/or spot sizeautomatically while maintaining the laser beam position accuracy.

There is also an increasing need to change the size of the field of view(FOV) of the machine vision system during operation to meet differentapplication requirements. Three-dimensional calibration on machinevision allows the system to change the size of FOV automatically andmaintain the vision dimension accuracy at the same time.

Referring to FIGS. 11A–11D, in one arrangement, a two-dimensionalcalibration procedure relating a position of the first side to the lasermarking field 24 on the second side includes a calibrated machine visionsub-system 14 and calibrated x-y stage 18 that will mark a mirror 92(one mark shown as 95 in FIG. 11B). A description of the calibration ofstage 18 and camera sub-system 14 is shown below (steps 1 and 2). Thetest mirror is positioned at a predetermined working distance withcoated surface facing the laser source. The marking laser beam 93 isdirected to several locations on the surface so as to mark 95 an N×Ngrid on the mirror 92. In the illustrated embodiment the x-y stage 18moves the mirror in both x and y directions so that the alignment visioncamera 13 can “see” each node on the grid from non-coated surface of themirror (opposite side from laser source). Illumination from light source21, or other suitable illumination, is used and depicted by illuminationbeam 94. The coordinates of each node are recorded. A calibratedalgorithm or look up table is then generated relating the coordinates.

The calibration techniques described herein are not restricted to“topside” imaging and “bottomside” marking. For example, the process maybe applied to wafer marking in a system where a chuck holds the wafer ina vertical position, and the marking and illumination beams aresubstantially horizontal. Likewise, the workpiece may be marked from thetopside based on calibration and reference data from the bottom-side.Similarly, the process may be adapted for calibrating separatedalignment and marking fields, both covering regions of a single side ofa workpiece.

In order to optimize the system for different application parameters,sometimes one or more machine settings might require adjustment duringthe operation. When the change in setting affects the system accuracy, anew calibration will be required. The three-dimensional calibrationprocess is used to create multiple layers of calibration files withrespect to different system settings. A three-dimensional calibratedsystem can switch between different settings automatically and achievethe required performance and accuracy by using the correspondingcalibration files. Exemplary methods to achieve three-dimensionalcalibration for different settings on the system include:

1. Laser beam spot size versus laser working distance: Use z-stage 28,and/or a combination of relative motion of chuck 17, and/or motion of anoptical sub-system within marking head 19 to relatively position thetest mirror to different working distances with respect to the lasersource. Varying the working distance de-focuses the laser beam andprovides different spot size at the work surface. It has been determinedthat a defocused spot provides acceptable mark quality for certainworkpieces, and hence is considered. The two-dimensional calibrationdescribed above is repeated for each working distance. As the result, agroup of calibrated algorithms or look up tables for different spotsizes with corresponding working distances is generated.

2. Laser beam spot size versus laser beam expander setting: Use anexpander for focus control, zoom expansion control, or the combination.For instance, a computer controlled embodiment of the expander 49 shownin FIG. 4 may be used to achieve different laser beam spot sizes on awork surface at fixed working distance. Different combinations of laserbeam expansion and focus can be used to achieve a desired spot size.Then the two-dimensional calibration described above is repeated foreach beam expander setting. As the result, a group of calibratedalgorithms or look up tables for different spot sizes with correspondingbeam expander settings is generated.

3. Laser beam working distance versus laser beam expander setting: Usean expander for focus control, zoom expansion control, or thecombination. For instance, a computer controlled embodiment of theexpander 49 shown in FIG. 4, may be used to achieve same laser beam spotsizes on a work surface at different working distances. The laser beamfocus relative to the work surface could be held constant or could varyby using different expansion settings while keeping the same spot size.Then the two-dimensional calibration described above is repeated foreach beam expander setting. As the result, a group of calibratedalgorithms or look up tables for different working distances withcorresponding beam expander settings is generated.

4. Machine vision field of view versus vision lens/camera setting:Adjust the zoom and focus on vision lens/camera 13, 15 of sub-system 14to achieve different sizes of field of view on a work surface. Repeatand generate a calibration algorithm or look up table for each visionlens/camera setting. As the result, a group of calibrated algorithms orlook up tables for different fields of view with correspondinglens/camera settings is generated. On an alternative arrangement,“software zoom” capability provides for a useable range of operationwithout requiring moving parts. In yet another arrangement the digitaland optical techniques may be combined.

In a preferred arrangement capability will be provided for adjustment ofsystem parameters (e.g. laser beam working distance and spot size) whilemaintaining calibration in the presence of “sag” or workpiece warpage.The warpage may be significant relative to the depth of focus forsmaller spot sizes, particularly for thinner wafers or workpieces (e.g.300 μm thick, 300 mm diameter). In one embodiment the alignment visionsystem 14 (e.g. positioned relative to the first side) and markercoordinates may be calibrated with at least the following steps:

Step 1.

Camera Calibration:

Use a precisely made grid template 91 (shown in FIG. 11-D) to calibratethe fine alignment camera's pixel size over the field of view 25 to thereal world unit. This will compensate for geometric distortion of thelens system and other static errors. In an alternative arrangement asingle “point” target may be translated through the camera fieldproviding stage limited accuracy performance over the field 25, at theexpense of additional calibration time, but may eliminate a requirementfor the grid.

Step 2.

X-Y Table Calibration:

Use the fine alignment camera sub-system and x-y stage 18 to measure aprecisely made full field size grid, which approximates or matches theworkpiece dimension (e.g. largest workpiece to be processed with thesystem). This step will compensate for static errors (e.g. tolerancestackup), including non-linearity and non-orthogonality of the stages.

Step 3.

Marker Field Calibration:

Laser mark a full field size 24 grid on a mirror 92, as shown in FIG.11C. Use the calibrated fine alignment camera (from step 1) and thecalibrated x-y table (from step 2) to measure each mark 95 of the gridon the mirror over a marking field 24. This step will compensate forgeometric distortion of the laser scanning lens and Galvanometer systemand other static errors.

Step 4.

Three Dimensional Marker Field Calibration:

In order to compensate for wafer sagging and warpage, the wafer ismarked a plurality of levels along the Z-axis 26. Multiple marker fieldcalibrations may be required. In this case, relative motion of one ormore of the (1) stage 18, (2) marking head 19 or internal opticalcomponents, for instance expander components 49 of FIG. 4, (3) stage 28,or (4) chuck 17 provides for relative positioning of the marking beamand grid. The marking occurs at several pre-determined levels along theZ-axis 26. Step 3 is repeated for each level.

Step 5.

Three Dimensional Fine Alignment Camera Calibration:

In order to compensate for different wafer thickness, focusing of thefine alignment camera is set at some slightly different surface levels.The focusing operation may include translation of the fine alignmentsub-system 14 along the Z-axis, or by adjustment of lens system 15, orin combination. Similarly, a Z-axis stage may be used to translate theworkpiece. Multiple vision field calibrations may be required. In thiscase, fine alignment camera will focus at several pre-determined surfacelevels along the Z-axis. Step 1 is then repeated for each surface level.

The technique in Step 4 will also allow setting different spot sizes (byde-focusing) on the fly for different applications Various curve fittingmethods known in the art may be applied at each of the calibration stepsto improve precision. The technique in Step 5 can also be applied toregister the mark inspection camera 20 and fine alignment sub-system.For instance, the optical centerline 29 may be approximately aligned atsetup and the calibration procedure used to precisely register thesub-systems. This is desirable so that the inspected marks may bedisplayed with a mark overlaying the corresponding die (for visualinspection), for instance. Software will be programmed to select correctcalibration files for proper application.

SECTION 2—Feature Detection and Fine Alignment

In the GSI Lumonics WH4100 wafer marker, offered by the assignee of thepresent invention, a fine alignment vision sub-system correctsrotational or offset errors (X, Y, Angle) which are introduced when awafer is placed in the marking station. A manual “teach tool” allows theuser to train the system to recognize three non-collinear points on thewafer that is to be used for the correction. The operator selects threeregions of the wafer (e.g. three corners of the overall pattern 115 ofFIG. 12. During 4100 operation a positioner then positions the cameraover the wafer and a die corner is visually selected. A “vision model”of the region is generated using an iterative trial and error processwith various adjustments. For instance, lighting adjustments are used toenhance contrast so that an acceptable match (“model score”) is obtainedat each of the measurement locations. Manual evaluation of the resultsis required with the system. The model information is then used todetermine mark locations on the bottom side of the wafer.

The model 4100 is used to process wafers up to 200 mm in diameter usinga “full-field” backside laser marker (e.g.: marker field covers theentire wafer). However, future generation marking systems will requiremarking of wafers up to 300 mm, for example, with miniature die orpackages of finer dimensions (e.g. 0.5 mm). Also, smaller wafers mayalso be produced in the future with die sizes a fraction of amillimeter.

Referring to FIGS. 1A and 12A, in a preferred embodiment of a system ofthe present invention the die pattern layout 115 and locations for markregistration (e.g. reference data from the first side) are automaticallydetermined by pattern matching of circuit features across the wafer 3using a vision sub-system. Preferably, no operator intervention isrequired, or at least the intervention is substantially reduced. Incertain applications the number of regions to be analyzed may beincreased (beyond three) to improve estimates.

By way of example, FIG. 12A illustrates several features, which may beused in the matching process. Within a die 112 circuit features mayinclude pad 5 which may be an interconnect, but as illustrated may be alocal fiducial. Other features to consider include trace edge locations7, die outline 6, or corner 110 locations. As shown in FIG. 1D similarinformation may be obtained from a grid array of interconnects, forinstance the die edge 6 or location of the Grid Array ball centers 8.The former approach is preferable, if the contrast is high. However, ifthe contrast is low at the location 6 between the die edge and thesurrounding “street,” the grid array locations or other features may beselected for training (e.g. if higher in contrast). Similarly, thesystem may be trained to include the spacing 114 between the die. It iscontemplated that the average measured spacing between several die (e.g.average pitch) will be a reliable measure and easy to relate to anavailable “wafer map.” For instance, the average spacing may be measuredbetween every die and the results averaged. The available wafer mapprovides coordinates of the die within the pattern and associatedinformation for marking. Such information may be obtained by estimatingthe locations of die edges (e.g. least squares fit) near the corners, orwith the use of correlation techniques to match a grey scale or binaryimage of region 116, which may be defined from the corner locations.Other features which may be present include local fiducial(s) 113 (ifpresent), or identification marks (letters, codes, etc). Such featuresmay be used alone or in combination with the above.

Those skilled in the art of machine vision measurement and patternrecognition will recognize that a number of tools may be used to obtainthe information be used for the automatic teaching method. For example,the AcuWin vision software provided by Cognex is suitable for performingvarious internal “matching” operations. WO0161275, earlier cited herein,also teaches various automatic learning algorithms for use in a 3Dsystem for inspection.

In one embodiment, during the training operation, a wafer is loaded intothe system after the pre-alignment step. The algorithm then determinesat least one of three regions for training based on wafer mapinformation. The region information will often be replicated over thewafer, so a single pattern may apply to the entire wafer. Preferably,the system is calibrated with the 2D/3D calibration process prior toteaching, but a complete calibration may not always be required.Referring to FIG. 2A, the wafer 11 (corresponding to 3 of FIG. 1A) andalignment vision system 14 are relatively positioned to view the region.Feature detection algorithms are executed, ultimately producingcoordinate locations for the die (and the backside marks). Preferably,the contrast between the image features is also automatically controlledby lighting or focus adjustments to improve performance. Methods forfocus and illumination control are well known in machine vision andnon-contact optical metrology. Preferably, the process is repeated ineach region to obtain performance statistics for various features thatmay be ranked and selected accordingly for marking subsequent wafers.

FIG. 12A shows a view of the of the front side, with a notch 604 (oralternatively, a flat as shown in FIG. 12A) at the bottom of a typicalwafer. In order to generate this transformation for each wafer atrun-time, a minimum of three points that are easy to locate and span areasonably large portion of the wafer surface area are to be selected.In at least one embodiment of the present invention, a position that canbe calculated based on qualitative information is associated with thepoint (such as die corner—upper-left, upper-right, lower-left, orlower-right—and die row and column number). FIG. 12A shows threeexemplary dies 602,601,603 which may be used. The expected location ofeach point is calculated based on the information, and may be used toconstruct a “theoretical polygon” that is substantially aligned to themovement of an XYZ Stage. At run-time, prior to processing each wafer,pattern-recognition software is used to determine the actual coordinatesof these three points on the wafer as it sits in the nest. These pointsare used to construct an actual polygon that is aligned to the diepattern on the wafer. The polygons are then compared to obtain atransformation (e.g.: translation, rotation and/or scale) between thetwo coordinate systems. The table below contains basic the informationthat is to be generated for each point of any given part type before anywafers of that type are processed by the system.

# Generated Output Data for Each Point 1 Row and column number of theassociated die at that point. 2 The die corner used; upper-left,upper-right, lower-left, or lower-right. 3 A vision model of the areaaround the taught point. 4 Coordinates of the point in the “primary”coordinate system.

The purpose of the FineAlignment training procedure is to generate thisinformation for a particular part type. The table below containspreferred information about a part type that is to be entered into thesystem before training can begin.

# Input Data for Each Part Type 1 The number of rows and columns ofactual dies on the wafer. 2 The X and Y pitch of the dies on the wafer.3 The X and Y die size. 4 The size of the wafer.

Referring to FIGS. 12A–12D, preferably, in order to generate theinformation shown in the output data table for each point, any portionof any die may be positioned at the center of the fine alignmentcamera's field of view. The location of the die pattern 115 on the waferand the orientation of the die pattern coordinate system 605 relative tothe “primary” coordinate system having origin 607, which is aligned withthe movement of the XYZ Stage.

Three pieces of information are sought:

-   a. The coordinates 610 of a point (x1,y1) in the primary system on    the left edge of the die pattern bounding box 606;-   b. The coordinates 611 of a point (x2,y2) in the primary system on    the top edge of the die pattern bounding box; and-   c. The rotation of the die pattern coordinate system 605 relative to    the primary coordinate system 607.

With this information the location of the upper-left corner 606 of thedie pattern bounding box in the primary coordinate system may bedetermined. The origin of the die pattern coordinate system is then adie_pitch_y up and a die_pitch_x to the left of that as shown. With theposition and orientation of the die pattern coordinate system known, thestage may be moved relative to any die location on the wafer.

Upon determining the locations of two actual die corners along the leftand top edges of the die pattern, and with capability for positioningany die location in the field of view, a search is performed (e.g.:search up/down and left/right) from these two corners looking for thelast die in each direction. The target dies for this algorithm are602,601,603 in FIGS. 12A, 12B and FIG. 1A. Each point is then chosen asone of the four corners of each die. In order to ensure the uniquenessof the area surrounding each corner, the lower-left corner of die 602,the upper-left corner of die 601, and the upper-right corner of die 603would be selected.

A vision model is to be generated in the area around each corner(including at least a portion of all four neighboring die locations).The model may include various features corresponding to the model ofFIG. 12A (e.g.: corners, edges, etc.) The data for all three points isstored for later retrieval by part type, to be used at run-time forprocessing all wafers of that part type.

Various alternatives may be used to practice a semi-automatic orautomatic training algorithm. For instance, additional die may beselected throughout the wafer and least squares fitting done to improveestimates.

An overall fine alignment process may be semi-automatic, but with analgorithm for automatic measurement of the die pitch with enhancedaccuracy. By way of example, the process may begin with a wafertransport tool moving a wafer to the nest. A user interface and displayallows an operator to move a wafer stage 18 of FIG. 11A (oralternatively a marking head with the wafer held stationary) to locate adie near the center of the wafer. A pattern, for instance similar tothat shown in FIG. 12C, is selected which will be used for the alignmentprocess. An image of a wafer portion is displayed and featuresidentified, for instance the lower corner of a die. A selected regionfor “teaching” may be evaluated for automatic recognition and thelighting adjusted as indicated for the WH 4100 system previously offeredby the assignee of the present invention. Commercially available patternrecognition software may be used, for instance the Cognex AcuWin visionsoftware.

In at least one embodiment the die pitch is measured prior to setting upthe at least second and third alignment locations or the at least threelocations 601,602,603 used to transform coordinates. The operator mayposition the stage and view the wafer to identify a suitable row of dieand further identify die corners, for instance the lower left and upperleft corner of a die. The stage may then be moved (e.g.: interactively)to the next die and a corner location identified from which the diepitch in a first direction is estimated. The process is then repeated inthe orthogonal direction.

Preferably the estimate is improved using a program to obtain additionaldata by traversing the wafer along rows and columns, identifying useabledie, and locating features (e.g.: corners) of the die with a patternrecognition algorithm. The data may be obtained at each row or columnwhere useful data is available, or in larger increments. The averagespacing may be estimated and related to a wafer map.

In a present system of the invention “ease of use” and minimal operatorintervention are considered beneficial improvements. Operator inputs maybe valuable to verify a column of die are useable, for instance. In oneembodiment the operator may verify that a selected die corner is useableand in a “topmost” column.

The additional locations for pattern matching are the selected, thestage positioned, and a test to verify the correct pattern recognitionsoftware operation.

SECTION 3—Workpiece Chuck/Positioner

It is desirable to grip and hold workpieces of varying shapes for theapplication of second side marking based on first-side data. Similarly,a preferred arrangement can also be adapted for general “double sided”laser processing and/or inspection operations.

Generally, at least one workpiece positioner is provided to relativelyposition the workpieces, and configured so as to support and positionworkpieces of varying specified dimensions. The arrangement allowsradiation beams to directly irradiate the first and second sides of theworkpiece over a large working area. Further, damage to the workpiece isavoided which might result from mounting on a fixture. Still further, adesirable arrangement allows for a robot driven end effector to load aworkpiece without movement of chuck.

In at least one embodiment a method and system for edge chucking andfocusing populated and blank silicon wafers of variable diameters andthickness is used. The method and system may also be used for otherapplications, for example in a micromachining process where a radiationbeam is to irradiate both sides of the workpiece.

FIGS. 7A–7D illustrates four views of a positioner (top, end, and sideviews 7A–C, respectively, and perspective view 7D).

The “chuck” system includes one or more positioners for supportingworkpieces of varying sizes, and for fine positioning of the workpiecewith one or more degrees of freedom. The chuck system is mechanicallycoupled to the X-Y translation stage 18 of FIG. 2A or other systemcomponents. Referring to the side view of FIG. 7C, a positioner includesa first axis drive 55 (linear stepper motor illustrated), a horizontallinear drive. It is to be understood that the drive may be achieved byvarious methods: e.g. 1. two position, open loop system such aspneumatic cylinder; 2. multi-positional, closed loop system such as alinear stepper or servo and guide. The pneumatically driven method maybe the lowest cost alternative, but provides less positionalflexibility. The first axis drive is used to position a second vertical(or normal) linear axis (again achievable through various methods) inthe correct location to hold the workpiece. A link 52 between axesprovides the coupling. The second, normal or vertical drive 54 is usedto position the workpiece at the correct height and orientation (e.g. aplane relative to an X-Y-Z coordinate system) to be in focus to andirradiated by a marking, inspection, or other radiation beams. Attachedto this second axis drive 54 (rotary stepper 57 with lead screw andlinear guide rail 58 shown) is a holding or “chucking” mechanism 51. Byway of example, the workpiece clamping mechanism of FIG. 7C is apneumatic rotary actuator 51 with clamp arm 59. Alternatively, thearrangement may be any combination of vacuum and positive mechanicalclamping (such as a pneumatic rotary actuator and a support base). Thesupport base 53 may optionally have vacuum ports, or a base with vacuumand no clamping device, for holding the workpiece while it is positionedand subsequently irradiated or inspected. In FIG. 7C a workpiece supportbase 53 is shown without vacuum ports. The perspective view in FIG. 7Dillustrates the shape of the support base.

The workpiece positioner (e.g. positioning sub-system) may beconstructed as shown in FIGS. 8A–8D to hold and adjust rectangularworkpiece 61 using two positioners 62, 63 each having the constructiondescribed above.

A chuck configuration utilizing, but not limited to, nor requiring,three positioners, driven by closed loop linear steppers or servos, isthe preferred method for holding most workpieces. FIGS. 9A–9C illustratean arrangement with three positioners 66, 67, 68, each which may havethe construction above, and an exemplary round workpiece 64, which maybe a Silicon wafer (e.g. 100, 200, or 300 mm diameter). The wafer istransferred with end effector 69 which is a component of a robot loadingtool used in a semiconductor manufacturing process, for example.

In operation, under control of a computer program, the workpiece isloaded by adjusting the distance between support 53 with the first axisdrive(s) to match the width of the workpiece. At least the height, andpreferably the attitude is controlled with the additional axis. Thisgenerally provides, when used in combination with other systemcomponents, at least five axes of adjustment (e.g.: X,Y,Z, roll, pitch).Further, the adjustment may be dynamic and occur during the laserprocessing operation or during idle periods.

SECTION 4—Laser Parameters and Mark Quality

It is desirable to produce high contrast, machine-readable marks, athigh speed in a designated region (e.g. specified by length, width, anddepth). Further, conformance to industry specifications prohibitsdamaging or otherwise adversely affecting the function or operation ofthe articles (e.g. a semiconductor device).

FIG. 10A illustrates an embodiment which can be applied for various highspeed workpiece 77 marking applications. Pulses generated from aQ-switched Vanadate Laser 71, having a typical output wavelength of1.064 μm, are shifted by wavelength shifter 72 to a shorter wavelengthfor efficiently coupling the energy into the workpiece. For wafermarking a frequency doubling crystal will produce a wavelength output atabout 532 nm. The optical switch 73, typically an acousto-opticmodulator, is computer controlled to allow pulses to reach the workpiece77 on demand. The motion of the workpiece mounted on stage 79 and X-Ygalvanometer deflectors 75 is coordinated by the computer. U.S. Pat.Nos. 5,998,759 and 6,300,590, assigned to the assignee of the presentinvention, teach various aspects related to “pulse on demand” controltechniques using a high speed optical switch as applied to semiconductormemory repair. Beam positioning accuracy of about 0.3 μm is typicallyachieved for cleanly removing semiconductor links.

Preferably the laser output will be generated from an Neodymium Vanadatelaser with a wavelength of 1064 nm for processing metal basedsubstrates. The output will be frequency doubled using the secondharmonic generator 72 to be 532 nm for non-metal substrates (e.g.silicon or gallium arsenide).

When practicing the present invention various alternatives may be usedfor pulse energy control, for instance, controlling (pulsing) the pumpdiode power for “marking on demand” with a series of pulses. U.S. Pat.Nos. 5,854,805, 5,812,569 describe such methods as applied to workpieceprocessing. A method of pulse control in laser systems is also describedin U.S. Pat. No. 6,339,604. Various combinations of pump, q-switch, andoptical switch controls may also be of benefit for controlling theenergy output, improving reliability, etc.

In a preferred embodiment for marking, a telecentric lens 76 and opticalsub-system 74 are used to control the spot size and distribution, whichpreferably will include optics for varying the spot-size and focusposition under computer control.

In an application to laser marking output pulses are produced having aset of pre-determined pulse characteristics including a repetition rate(and corresponding temporal pulse spacing), pulse width, and outputenergy.

Selected pulses gated by the switch 73 or otherwise controlled (whichmay be a “burst” or “string” of pulses) irradiate the wafer 77 surfaceat a first predetermined marking location within the marking field ofthe mirrors 75. The stage 79 may be a step and repeat stage used whenthe workpiece is larger than the marking field (e.g. as also illustratedfor the “second side” case of FIG. 2A). Referring to FIG. 10B, a laserpulse penetrates the wafer surface (e.g. silicon) within a depth rangesufficient to produce a machine readable mark 781 at the markinglocation. Damage to the wafer is avoided by limiting the depth ofpenetration 782 (as might be measured by the 1/e energy level) withcontrol of the pulse characteristics, for instance the peak energy andpulse width. Deeper penetration 784 results in a crack. In a preferredsystem the laser energy at 532 nm will be absorbed at a maximum depth of10 μm in a typical silicon substrate. This control prevents microcracking 783 and other hazardous effects inside the substrate (e.g.bubbles). The step of irradiating is repeated at a plurality of markinglocations.

Preferably, the pulse width will be within a range of about 10 to 15nsec to produce a mark with sufficient contrast.

The energy per pulse incident on the surface is preferably in the rangeof 0.00023 to 0.00025 Joules (eg: 230–250 microjoules) produces highquality marks on Silicon wafers.

Preferably, the marking speed is improved to a higher linear speed onthe wafer surface 77, with a relatively high pre-determined pulsefrequency of the laser 71. By way of example, a repetition rate of about15–30 KHz, for instance 25 KHz, provides significant improvement overearlier wafer marking systems used at both near Infrared and Greenwavelengths. With a preferred spot size of about 30–35 μm, linearmarking speed greater than 150 mm/sec is a relative improvement overprevious wafer mark systems. A speed of about of 350 mm/sec is expectedfor use in a system having the preferred laser pulse characteristics.Reduced solid state laser power at high repetition rates constrainedearlier performance, and separation of spots on the surface wereobserved which limited mark quality.

A laser pulse is focused into a spot diameter to produce energy densitywithin a predetermined range. The minimum distance between a pair ofmachine readable marks may be further reduced by controlling the spotdiameter with optics 74. Such an arrangement may include a “zoom” beamexpander in 74 or other optical components which areremovable/insertable, preferably under computer control (e.g.: as shownin FIGS. 4 and 6). The spot size adjustment is generally desirable tocontrol the mark linewidth and contrast. A spot diameter in a range ofabout 30 to 35 μm and a working distance to the workpiece of about 220mm to 250 mm represent exemplary ranges of operation. The smaller spotsize provides improved capability for producing higher mark densitycompared to earlier marking systems, and higher speed is provided withthe pulse characteristics.

Results for backside marking of Silicon wafers have shown the depthrange of a mark is to be about 3 to about 4.5 μm so to produce a machinereadable mark 781 with enough contrast to the background. The result wascontrary to an expectation that larger penetration depth was required.The results also provided additional margin for avoiding damage.

FIG. 14 shows a top view of a mark 950 to illustrate measured variationof average marking depth 951 with various laser parameters. The heightin the table below represents material 952 on the side of the markresulting from removal of material by melting. The average depthvariation measured with an interferometer illustrates exemplaryperformance with laser power and repetition rate at various markingspeed. The 100% rating allows for an estimate of maximum performance.The following data was obtained:

Rep. Laser Power Rate Mark Speed Average Mark Average Mark (% max.rating) (KHz) (mm/sec) Height (μm) Depth (μm) 80 20 120 4.36 −4.75 80 20200 4.53 −4.43 80 20 300 4.65 −5.61 100 10 120 3.58 −5.40 100 10 2003.41 −4.33 100 10 300 3.64 −2.90 100 20 120 4.08 −9.91 100 20 200 3.58−6.45 100 20 300 3.55 −4.53

Further analysis of the marks indicated sufficient contrast for machinereadability over a range of about 3–4.5 μm. Increasing the mark depth tothe larger numbers, for instance 9.91 μm, produced cracking.

An interferometric scan was obtained of a wafer marked at a repetitionrate of 10 KHz and a marking speed of about 120 mm/sec. Severe crackingas exemplified by the “spiky” data which results from structuralvariations at a depth of about 9 μm or more. Scans also showed goodresults with maximum depth of about 4 μm.

The shifted wavelength may be below the absorption edge of the workpiecematerial, but need not be restricted to 532 nm. For instance, theworkpiece may be Silicon wafers or metal. The wavelength will preferablybe substantially less than the absorption edge of Silicon (1.12 μm) formarking in accordance with the present invention.

Suitable lasers may include commercially available diode pumped (DPL)Nd:YAG lasers with about 6 Watts IR output, and output 3 Watts in theGreen. An alternative, though more expensive, is a 10 Watt (W) DPL laserwith about 10 W IR and 5 W green output power. Preferably, the opticalsystem will contain high efficiency optical components to minimizelosses.

The Vanadate laser is preferred for marking Silicon wafers, but is notessential for practicing the invention. The desired pulsecharacteristics may be implemented with other designs, provided allspecifications (e.g. beam quality, stability) are met. For instance, afiber optic amplified system (e.g. Master Oscillator Power Amplifier)may be used to produce short pulses at relatively high rates. A solidstate laser, including a fiber laser, with a slower repetition rate butsufficient power may be “pulse stretched” with a delay line and beamcombiner(s) to increase the output repetition rate of the laser system.

SECTION 5—Precision Teletronics Lens

In precision laser marking and other similar material processingapplications, for instance embedded resistor trimming, there is a needto produce fine spot sizes so as to control the width and contrast of amark (or kerf) while maintaining precise spot placement over arelatively large 3-dimensional field. For instance, a 300 mm wafer mayhave die sizes ranging from about one millimeter or less with a tightlyconstrained marking region defined within the die. A spot size of about30 μm will produce high contrast marks, but the depth of focus is aboutfour-times less than that of earlier marking systems. For thin wafersthe warpage may be a significant fraction of the depth of focus, so thethree-dimensional spot size/spot placement considerations are valuable.

In non-telecentric scanned laser systems, spot placement errors at aworkpiece plane will vary with depth, and may significantly degrade thesystem accuracy. Such z-axis error may be the result of workpiece tilt,defocus, sag, warp or any deviation from an ideal target plane. Fornon-normal incident angles of the scanned beam, the z coupling isapproximately the deviation angle from normal incidence times the localz error. Preferably, a telecentric scan lens is used for focusing themarking laser onto the field. The telecentric scan lens, well known inthe field of laser scanning, is used to maintain a near normal incidenceangle of the beam to the workpiece thereby minimizing z coupling and theresulting x and y position errors. The approximate invariance of angleover the field may also have other advantages, such as providing forcoaxial detection of reflected radiation. Coaxial detection can be usedwith many know methods to determine focus position, for exampleastigmatic spot detection.

Considering the first order scan lens properties, placing the scanorigin at the front focal plane of the lens will produce a telecentricscan. In practice, there are non-linearity errors in the lens designthat deviate from perfect telecentricity. Those skilled in the art ofscan lens design recognize that correction of these errors is possibleby modifying the individual lens elements and/or adding additionalelements to the lens design.

Typically, an x y galvanometer scan system has two scan mirrors, asshown in FIG. 4. A distance sufficient to prevent physical interferenceand beam occlusion separates the mirrors. The mirror separation createsdifferent scan origins for each axis and therefore prevents both axesfrom being located at the lens front focal plane. Often, the focal planeis placed at an intermediate position. This creates an additional fielddependent telecentricity error, based on the mirror locations and thelens focal length. In a typical system the error may be 1 to 2.5 degreesat the worst field point. Various techniques are useful for correctingtelecentricity error, for instance as described in U.S. Pat. No.4,685,775 by Goodman, which is hereby incorporated by reference in itsentirety. A beam translator improves the correction.

With field dependent error, a portion of the field may be selected toreduce errors at the workpiece. For instance, a small central portion ofthe field is used and material is processed with improvedtelecentricity. With one scan mirror located near the front focal planeof the lens, a first axis of the field addressed with this mirror willhave better telecentricity than a second axis addressed by a secondmirror more remote from the front focal plane. In this case, a portionof the field having improved telecentricity may be selected with alarger dimension along the first axis and a smaller dimension in thesecond axis, for example a rectangular field. It is also recognized thatby using a rectangular field, the first axis may be larger than the edgea square field. Selecting a portion of the field may reduce other fielddependent errors such as thermal drift of X-Y galvanometer deflectors.For example, a quadrant of the field where gain drift is mitigated inpart by offset drift in each galvanometer may be selected to reducebeam-positioning errors.

For embodiments using through the lens viewing, the scan lens istypically required to image a target at wavelengths other than theprocessing wavelength. Color correction elements can be used in a designto improve viewing performance. Telecentric scan lenses with colorcorrection for through the lens viewing are know, for instance the scanlens used in the commercially available GSI Lumonics Model W672 lasertrimmer.

A preferred embodiment for precision laser marking of large wafers andsimilar applications includes a three-element telecentric scan lens 990as shown in FIG. 13A. This lens has an effective focal length of 155 mmat 532 nm and is capable of forming 30 micron spots over a scan field of80 mm square. The total path length is about 360 mm. With uncorrected,spaced mirrors the telecentricity error is approximately 2 degrees.FIGS. 13B and 13C show the telecentricity error 991 and 992 across twoorthogonal scan axes. In both cases the error has non-linear variation.Over a depth range corresponding to wafer sag of +−300 μm, the worstcase spot placement error is about +−13 μm, slightly less than one spotdiameter.

In the precision marking system, wherein three dimensional tolerancesdetermine system performance, the spot placement accuracy of the lenssystem is to be maintained by including a method for three-dimensionalcalibration. In one embodiment the wafer is positioned with a workpiecepositioner so that a best fit plane (over the wafer) is aligned normalto the marking head. A location is then determined relative to bestfocus position of the telecentric system of FIG. 13A. The beampositioner is directed based upon the location of features and storedcalibration data.

At least one embodiment of the present invention may include a precisionscan lens with improved telecentricity when compared with a conventionalnon-telecentric scan lens. In one example, the maximum angle incident atthe workpiece may be less than about half of the maximum angle of thebeam incident on the scan lens entrance pupil. In another example, themaximum deviation angle to the workpiece may be limited to less thanabout 10 degrees. This type of scan lens can be smaller, and may be lesscomplex than a larger telecentric scan lens. Thus, a precision scan lenswith improved telecentricity may be used to provide a design compromisewith both a level of improved marking accuracy with changes in theworkpiece height and reduced lens size, complexity and cost.

SECTION 6

Backside Mark Visual Inspection with Frontside Die Registration

In early versions of certain backside wafer marking systems an infraredsource was used to “backlight” a wafer so as to view backside features.With high density circuitry increasing at a rapid rate, the “backlight”approach will not always be possible in the future.

In one embodiment of a wafer marking system used to form marks on thebackside of a wafer, an inspection feature includes a registered displayof the mark and die. In a preferred embodiment inspection feature usestwo cameras, one above and one below the wafer. FIG. 2A illustrates thecamera 13 of fine alignment vision system 14 registered along centerline29 with the mark inspection system 20. A satisfactory degree of imagematching between corresponding front and backside wafer portions may beachieved with manual adjustment at system setup, for instance. Systemcalibration may then be used to improve the precision.

In at least one embodiment of the system, the equipment calibrates thebottom camera system 20 to the top camera system. Preferably, thecameras are in fixed positions. One or more cameras may have a zoom lenswhich is manually adjustable. In one arrangement, a calibration targetof a transparent surface is placed between the two cameras. The image isacquired with both cameras. The images are superimposed and, usingpattern-matching software, for instance commercially available toolsfrom Cognex Inc, a correction offset, angle, and scale is calculated toalign the bottom camera's image to the top camera. FIG. 17A illustratesa calibration target, the image of which is to vary with offset, scaleand rotation. Various other commercially available or custom targets maybe used. The translation, scale, and rotation correction (includinginversion of a coordinate axis) is automatically determined in software.

During the inspection operation the top camera is used to acquire animage of the die on the topside of the wafer. The bottom camera is usedto acquire an image of the mark on the backside of the wafer. Bysuperimposing the coordinate systems of the two images, analysisdetermines the accuracy of the mark with respect to the die.

During inspection, this calibration data is applied to the mark image.Using pattern matching or OCR software the location of the mark relativeto the location of the die is known.

It is to be understood that this feature is not restricted to top andbackside wafer marking, but may be applied to any two sides or separatedfields.

Inspection of marks may be done on-line or off-line. The inspection mayinclude a random sample of die or up to 100% inspection. In at least oneembodiment an operator may setup a region of interest 900 within abackside image corresponding to at least a portion of a die as shown inFIG. 17B. Preferably, the operator will be able to adjust 901 the areaof interest, as shown in FIG. 17C, and make any necessary adjustmentsfrom a wafer map or with minor adjustments between die. A typical markmay occupy 50–60% of the area of a die, but up to about 80% is possible.

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention as defined by the following claims.

1. A method for inspecting machine readable marks on one side of a waferwithout requiring transmission of radiant energy from another side ofthe wafer and through the wafer, the wafer having articles which mayinclude die, chip scale packages, circuit patterns and the like, themarking occurring in a wafer marking system and within a designatedregion relative to an article position, the articles having a pattern ona first side, the method comprising: imaging a first side of the wafer;imaging a second side of the wafer; establishing correspondence betweena portion of first side image and a portion of a second side image; andsuperimposing image data from the first and second sides to determine atleast the position of a mark relative to an article.
 2. The method ofclaim 1 further comprising substantially matching images obtained fromthe first and second sides so that the superimposed image portionscorrespond, wherein the step of substantially matching is carried outusing a calibration target and a matching algorithm.
 3. The method ofclaim 2 wherein the superimposed data is used to determine the positionof a mark relative to the article.
 4. A system for inspecting machinereadable marks on one side of a wafer without requiring transmission ofradiant energy from another side of the wafer and through the wafer, thewafer having articles which may include die, chip scale packages,circuit patterns and the like, the marking occurring in a wafer markingsystem and within a designated region relative to an article position,the articles having a pattern on a first side, the system comprising:means for imaging the first side of the wafer to obtain an image; meansfor imaging a mark on the second side of the wafer to obtain an image;means for establishing correspondence between a portion of a first sideimage and a portion of a second side image; and means for superimposingimage data from the first and second sides to determine at least theposition of the mark relative to an article.
 5. The system as claimed inclaim 4 wherein the means for establishing correspondence includes acalibration target and an algorithm.